In the design of complementary metal-oxide semiconductor (CMOS) integrated circuits, differential amplifiers are used for various applications because a number of advantages can be derived from these types of amplifiers, as compared to single-ended amplifiers. Differential amplifiers are used to amplify analog, as well as digital signals, and can be used in various implementations to provide an output from the amplifier in response to differential inputs. For example, a general-purpose differential amplifier amplifies the difference of the two input signals. But these differential amplifiers can be readily adapted to function as an operational amplifier, a comparator, a sense amplifier and as a front-end buffer stage for another circuit, etc. Differential amplifiers are utilized where linear amplification having a minimum of distortion is desired. A fully differential amplifier differs from a differential amplifier in that its outputs are also differential. Using a fully differential design gives a designer a way to dramatically reduce the common mode noise present in the system and the second harmonic in the fast fourier transform of the output signal.
FIG. 1A illustrates a conventional fully differential amplifier. The fully differential amplifier shown in FIG. 1A is a two-stage amplifier with a common mode feedback CMFB controlling the first stage, load transistors M3 and M4. A common mode feedback input CMIN signal is supplied by the resistor/capacitor chain: R1, R2, C1, and C2. In some applications such as switch capacitor networks, there can be a state where the amplifier inputs are connected with its outputs, as illustrated in FIG. 1B. If for some reason there is positive supply (VCC) voltage on both outputs VOUTP & VOUTN, a common mode feedback amplifier AMPCM will be saturated, both load and input transistors M1–M2 of the main amplifier are in a cut off state, and the nodes n1, n2 are in an undefined condition. Under this condition, any small leakage to ground on these nodes will cause the state to be stable and the amplifier may never get out of this state. As such, the amplifier will be in a stable, but undesired state of operation.